Harrows of various types have been known in the art for many years. Typically, such harrows utilize gangs of concavo-convex discs or spike wheels to loosen and finish the seed bed soil. Disc gangs advantageously lift and move soil to the side to some extent as they penetrate and loosen the soil, but discs have the disadvantage of tending to create a compaction layer that is difficult for plant roots to penetrate.
The present invention provides a spike wheel that avoids the creation of a compaction layer yet still has the ability to dislodge and move soil laterally to a certain extent as it loosens and lifts the soil while crushing clods and partially burying residue. In a preferred embodiment, the spikes of each spike wheel radiate outwardly from a hub of the wheel and are provided with outermost tips that are slightly inturned so as to yield a generally concavo/convex overall configuration for the wheel. Thus, although the spikes penetrate, loosen and dislodge the soil without creating a layer of hardpan, their slight concavity gives them the ability to lift and move soil laterally to a certain extent as the wheels are arranged in obliquely disposed gangs and drawn across the surface of the ground.
To facilitate manufacture, installation and replacement of the spike wheels, all wheels of each gang are identical and are mounted on a common shaft having a circular cross section. In order to maintain adjacent wheels slightly rotatively offset from one another so that corresponding spikes of adjacent wheels to do not engage the ground simultaneously, adjacent spike wheels in the gang are provided with mutually interengaging structure that holds the adjacent wheels slightly indexed relative to one another. In a preferred embodiment, such structure takes the form of one or more projections on the hub of one wheel and one or more corresponding receiving notches on the hub of the adjacent wheel. Each wheel is thus provided with one or more projections at one end of its hub and one or more notches at the opposite end of the hub, such notches being angularly offset from the projections.